The present invention relates to the art of earth boring and more particularly to an earth boring cutter with replaceable cutting elements.
Conventionally, rotary drilling apparatus and particularly rotary drilling apparatus for boring large diameter holes and tunnels includes a multiplicity of roller cutters. A number of the roller cutters together with their bearings and the saddles in which the cutters and bearings are mounted are positioned on a rotary head or a bit body and used to form holes in the formation being bored. The roller cutters may be conveniently mounted on the rotary cutting head of a tunneling machine or on the body of a raise bit.
A disk type roller cutter is effective in very soft to medium formations. The disk is usually a heat treated alloy steel cutter with an included angle between about 60.degree. and 90.degree.. The disk cutters are usually indexed two or three inches apart. In effect, the disk cutters plow concentric circles around the face of the formation being bored. The cutting disks are indexed so that the formation between disks will break out completely under a given load and R.P.M. This is a very efficient way to cut formations because the cuttings come off the face in relatively large pieces. Disk cutters are not economical in harder formations because the disks dull out quickly in the harder abrasive formations. This is especially detrimental in shaft drilling or raise drilling operations where trip time is costly. It is not practical to make the disks completely of carbide and the brazing on of continuous sintered carbide tips or wedges is also of questionable feasibility from an economical and operational standpoint. Since the bearing life of the roller cutters long outlasts the life of the cutting structure, the cutting structure should be replaced periodically thereby extending the useful lifetime of the cutter. It generally is difficult, expensive and time-consuming to remove the entire cutter in order to replace the cutting element. The cutters are heavy and are often in a position that makes removal of the entire cutter awkward and impractical. For example, it is difficult and time-consuming to remove the cutters on the cutting face of a tunneling machine during a tunneling operation.